This invention relates generally to thermally conductive adhesive material and manufacturing methods and more particularly to thermally conductive adhesive material and manufacturing methods for bonding integrated circuit (IC) packages to the surface of a printed circuit board (PCB). (As is known in the art, an integrated circuit package is typically mounted on a printed circuit board with an adhesive material disposed between the IC package and the surface of the PCB. The adhesive material serves the dual purpose of bonding the IC to the printed circuit board and dissipating heat generated by the IC into the printed circuit board. The thermally conductive adhesive material is usually electrically insulating to prevent, for example, a short circuit from occurring between IC leads which contact the adhesive material. One type of thermally conductive adhesive material which has been used is an epoxy-based resin material,) such as Ablefilm 506.TM. epoxy, manufactured by Ablestik Laboratories of Gardena, California. While such epoxy-based adhesive performs satisfactorily in some applications, often the adherence properties (i.e. the bond strength between the epoxy and the IC package or PCB) thereof are too strong to allow removal of the IC package without damaging the device. For example, typical thermally conductive epoxies have bond strengths of about 2,000 psi. Further, such epoxy-based adhesive materials when cured have a relatively high modulus of elasticity (i.e. Young's Modulus) under both compression and tension stress, well above 500,000 psi at room temperature. As is known, the modulus of elasticity of a material is the ratio of stress applied (either in compression or tension) to the material to the strain induced thereby (in compression or tension, respectively). Materials having a high modulus of elasticity, such as epoxy, are quite rigid and thus do not deform in response to thermal expansion or contraction of the IC or PCB. Since there is typically a mismatch between the thermal expansion and contraction characteristics of an IC package and a printed circuit board, the rigidity of the epoxy-based adhesive material disposed therebetween may cause damage to the IC package, separation of solder joints between the IC and PCB, or cracking of circuit "runs" on the PCB in response either to the thermal expansion of the IC package and PCB against the rigid epoxy-based adhesive material in the presence of elevated temperatures or to the slight contraction of the IC package and PCB away from each other as occurs when such structures are cooled.
Another type of thermally conductive adhesive material is a "hot melt" adhesive, such as Adhesive No. 1670, manufactured by Rexham Corporation of Matthews, North Carolina. Such a "hot melt" adhesive is a wax-like material which is applied at elevated temperatures when the adhesive is formable, the waxlike adhesive hardening when cooled. However, such "hot melt" adhesives typically deteriorate when exposed to solvents of the type normally applied to the surface of a PCB to prepare the board for conformal coating. Silicone-based adhesive materials, which have a relatively low modulus of elasticity and thus are soft and pliable, are also available, but such silicone material swells during the solvent cleaning process described above. Also, silicone may be extracted from the adhesive material by the solvent and redeposited on the surface of the PCB, thereby causing poor adhesion of the subsequently-applied conformal coating to the PCB.
In addition, it may be desirable that the thermally conductive adhesive material have low outgassing properties, that is, a decreased tendency to evaporate small portions of the material in the form of gas molecules.